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Human reproduction Presented by Kalpesh gadhvi
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. Menstrual cycle is the reproductive cycle in all primates and begins at puberty (menarche) . In human females, menstruation occurs once in 28 to 29 days. The cycle of events starting from one menstruation till the next one is called the menstrual cycle. . During the middle of the menstrual cycle, one ovum is released (ovulation) . The cycle starts with the menstrual flow (3 to 5 days), caused due to the breakdown of the endometrium of the uterus. Blood vessels in liquid state are discharged, but this occurs only when the ovum is not fertilised.
. It is followed by the follicular phase.ln this phase, the primary follicles mature into the Graffian follicles. This causes the regeneration of the endometrium. . These changes are brought about by ovarian and pituitary hormones. In this phase, the release of gonadotropins (LH and FSH) increases. This causes follicular growth and the growing follicles produce oestrogen. The LH and FSH are at their peak in the middle of the cycle (14th day), and cause the rupture of the Graffian follicles to release ovum. This phase is called the ovulatory phase.
amus GnRH Anterior pituitary Mideycle peak of LH (triggers ovulation) FSH LH Blood levels of FSH LH levels LH Growth of follicle-Ovulation Corpus Oestrogen Progesterone Blood levels of Endometrium of uterus ogcstcronc Ocstrogen Menstruation days 4 hilicular phasco -al phase of menstrual evcle menstrual evcle
. The remains of the Graffian follicles get converted into the corpus luteum, which secretes progesterone for the maintenance of the endometrium. In the absence of fertilisation, the corpus luteum degenerates, thereby causing the disintegration of the endometrium and the start of a new cycle. . In humans, the menstrual cycle ceases to operate at the age of 50 years. This phase is known as the menopause.
Fertilization and implantation
During coitus, the semen is released into the vagina, passes through the cervix of the uterus and reaches the ampullary-isthmic junction of the fallopian tube. . The ovum is also released into the junction for fertilisation to occur. . The process of fusion of the sperm and the ovum is known as fertilisation. . During fertilisation, the sperm induces changes in the zona pellucidaand blocks the entry of other sperms. This ensures that only one sperm fertilises an ovum. . The enzymatic secretions of acrosome help sperm in penetration to ovum cytoplasm.
. This causes the completion of meiotic division of the secondary oocyte, resulting in the formation of a haploid ovum (ootid) and a secondary polar body. . Then, the haploid sperm nucleus fuses with the haploid nucleus of the ovum to form a diploid zygote. . Mitosis starts as the zygote moves through the isthmus of the oviduct (cleavage) and forms 2, 4, 8, 16 daughter cells called blastomeres. The 8-16 cell embryo is called a morula,which continues to divide to form the blastocyst. The morula moves further into the uterus.
Pregnancy, parturition and lactation
. The placenta is connected to the embryo by the umbilical cord. The placenta acts as an endocrine gland, and produces the human chorionic gonadotropins, human placental lactogen, oestrogen, progesterone and relaxin (later stages of pregnancy) . These hormones support foetal growth and help in the maintenance of pregnancy. Hormones like oestrogen, progestogen, cortisol, prolactin, etc., are increased several folds in the maternal blood. . Immediately after implantation, the inner cell mas:s (embryo) gets differentiated into the ectoderm, mesoderm and endoderm, which give rise to the different tissues. This ability of the inner cell mass is due to the presence of multi-potent cells called stem cells.
. This causes the release of oxytocin from the pituitary, which causes stronger uterine contractions. . This leads to the expulsion of the baby along with the placenta. . During pregnancy, the mammary glands undergo differentiation, and milk is produced during the end of pregnancy . The milk produced during the first few days of lactation is known as colostrums.lt contains several antibodies that aid the newborn to develop resistance.